I had a check engine light come on due to a o2 sensors which were giving a slow response but this was actually caused by a leaking flexi pipe which had been there for some time.

This case can lead to incorrect air fuel ratio which increases fuel consumption.

The leak has been there for a while but the light only came on much later. Would there have been poor fuel consumption gradually from earlier or does the computer only change the air fuel ratio when the light comes on. Car never felt sluggish to me until I saw the light.

Also what if the o2 sensor had been the problem(not the leak), would fuel consumption get worse gradually or does the computer only know when the light comes on?

2 Answers
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Looking at the context of your post, I'd guess you are asking if 'fuel consumption will increase' gradually before a check engine light turns on.

Then the answer to that is yes it is possible. The U.S. government has made it mandatory for the MIL (Malfunction Indicator Light) to come on when emissions exceed the limit of 1.5 times or 150%. That is still quite a bit of wiggle room for various engine performance issues to slowly arise without ever seeing the MIL come on..

The fact that your vehicle only felt sluggish once the MIL came on could be any number of possibilities, but I would say that the ECU (computer) went into some sort of FMEM (Failure Mode Effects Management) once it realized one of the sensors was not a reliable source of information anymore.

Oil consumption is not usually connected with air/fuel mixture. There are known operating conditions that do cause increased oil consumption and most of them are blatantly obvious. They are: hard accelerations, high speed passing, climbing steep hills or mountains, pulling heavy loads or large bulky trailers that offer significant wind resistance. Sustaining engine operation over 3,000 RPM's will increase oil consumption. There is one final factor: oil viscosity itself. 0W-20 tends to consume faster than 10w-30 in warm weather. Many users with engines over 100K miles reduce oil consumption with 15W-50. Large trucks are geared to operate at higher revs and tend to consume more oil than sedans or minivans. Speaking of exhaust flexipipe - be careful of fuel lines or wires near the pipe connections. A small exhaust leak blowing on a fuel line can cause vapor lock or burnt wiring.